1. Field of the Invention
The subject matter of this invention is related generally to molded case circuit breakers and more specifically to removable trip units for molded case circuit breakers.
2. Description of the Prior Art
Molded case circuit breakers are well known in the art as exemplified by U.S. Pat. No. 5,910,760 issued Jun. 8, 1999 to Malingowski et al., entitled "Circuit Breaker with Double Rate Spring" and assigned to the assignee of the present application. The foregoing is incorporated herein by reference.
Molded case circuit breakers include a set of separable main contacts, one of which is usually fixed and one of which is movable for automatically opening upon the occurrence of an overload or short circuit electrical current in the network which the circuit breaker is provide to protect. The separable main contacts are opened as a result of the functioning of a latched operating mechanism, which is interconnectable by way of an operating handle to a region outside of the circuit breaker. The operating handle may be used to trip the circuit breaker manually or to reset and close the circuit breaker contacts once they have been opened automatically. The reset action is required because circuit breakers must be mechanically charged to be in a state to reopen immediately upon closure in the event that the fault which cause the tripping in the first place has not disappeared. The reset action charges the circuit breaker for that purpose. Molded case circuit breakers have trip units, which are often removably insertable in the circuit breaker case. The trip unit in addition has at least two calibratable functions, one of which is generally identified as thermal tripping and the other of which is generally identified as magnetic tripping. The trip unit includes a rotatable trip bar, which when rotated will actuate a latchable tripping operation within the operating mechanism to automatically open the circuit breaker contacts. The rotatable trip bar is usually actuated in one of two ways. The first way is in response to what is called a magnetic tripping of the circuit breaker. This occurs when the amount of current flowing through the separable main contacts of the circuit breaker is so high as to represent a potential catastrophic failure and which therefore requires exceedingly quick opening action of the circuit breaker. In such a case a electro-magnetic core, which produces magnetic flux in proportion to the amount of electrical current flowing through the separable main contacts attracts a movable armature, the movement of which eventually causes the trip bar to move to thus cause the tripping action. The second tripping occurrence is in response to a relatively low amount of overload current, which eventually will cause overheating of the electrical wires in the circuit to be protected, but which does not necessitate the instantaneous action a short circuit requires and thus does not require the magnetic action spoken of previously. In this case a bi-metal element is heated by a heater element which conducts the electrical current flowing through the separable main contacts. As the bi-metal element flexes or moves it impinges upon the tripping bar causing it to flex and move correspondingly, until eventually a point is reached in which the tripping bar causes the circuit breaker to unlatch and trip automatically. Both the magnetic trip mechanism and the thermal trip mechanism usually require initial calibration.
In one half of an AC cycle, the electrical current flows through the circuit interrupter from the load by way of a terminal collar to the load terminal of the circuit breaker and from there into the trip unit where it flows through the previously mentioned heater which in turn is serially connected to the electro-magnetic member of the magnetic trip device. From there it is interconnected by way of a flexible cable to one end of a moveable contact arm and from there to the main contact on the moveable contact arm. When the contact arm is closed, it is closed upon a fixed contact which is supported usually on unshaped conductor, which in turn is interconnected with a line terminal and there to the line terminal collar and finally to the electrical line. In addition the circuit breaker usually has an arc chute for assisting in diminishing the electrical arc drawn between the separating contacts during the opening operation for extinguishing of the arc. The circuit breaker also has a slot motor arrangement, which is utilized to interact magnetically with the electrical current flowing in the opening contact arm to accelerate the opening of the contact arm magnetically. The operating mechanism usually consists of a series of levers and linkages, which are interconnected with the separable main moveable contact arm, the handle mechanism, and by way of a latch arrangement with the aforementioned trip bar. Description and operation of all of the above may be found in the previous mentioned, incorporated by reference '760 patent.
As was mentioned, each circuit breaker usually contains a trip unit which is integrated into the circuit breaker case during the construction process and which causes the tripping of a circuit breaker due to either a magnetic level current overload or a thermal level current overload. An example of such a trip unit may be found in the aforementioned U.S. Pat. No. 5,901,760. Utilization of a trip unit of this kind usually requires mounting of the trip unit from the front or faceplate side thereof. That requires that the area above the mounting screws for the circuit breaker be left open to allow access for a driver which often results in wasted space in the circuit breaker. It would be advantageous if a trip unit could be found in which this wasted space is no longer required and which could be placed into the circuit breaker very late in the manufacturing process, even after the main circuit breaker frame had been completed.